1. Field of the Invention
The present invention relates to an encapsulated toner for heat-and-pressure fixing used for development of electrostatic latent images in electrophotography, electrostatic printing, or electrostatic recording, and to a method for producing such an encapsulated toner.
2. Discussion of the Related Art
As described in U.S. Pat. Nos. 2,297,691 and 2,357,809 and other publications, conventional electrophotography comprises the steps of forming an electrostatic latent image by evenly charging a photoconductive insulating layer, subsequently exposing the layer to eliminate the charge on the exposed portion and visualizing the formed image by adhering colored charged fine powder, known as a toner, to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process).
As indicated above, the toner must meet the requirements not only of the development process, but also of the transfer process and the fixing process.
Generally, a toner undergoes mechanical frictional forces due to shear force and impact force during the mechanical operation in a developer device, and deteriorates after copying from several thousands to several ten thousands of sheets. The deterioration of the toner can be prevented by using a tough resin having such a high molecular weight that it can withstand the above mechanical frictional forces. However, this kind of a resin generally has such a high softening point that the resulting toner cannot be sufficiently fixed by a non-contact method, such as oven fixing or radiant fixing with infrared rays, because of its poor thermal efficiency. Further, when the toner is fixed by a contact fixing method, such as a heat-and-pressure fixing method using a heat roller, which is excellent in thermal efficiency and therefore widely used, it becomes necessary to raise the temperature of the heat roller in order to achieve sufficient fixing of the toner, which brings about such disadvantages as deterioration of the fixing device, curling of the paper and an increase in energy consumption. Furthermore, the resin described above is poor in grindability, thereby remarkably lowering the production efficiency of the toner. Accordingly, the binder resin having too high of a degree of polymerization and also too high of a softening point cannot be used.
Meanwhile, according to the heat-and-pressure fixing method using a heat roller, the surface of a heat roller contacts the surface of a visible image formed on an image-receiving sheet under pressure, so that the thermal efficiency is excellent and therefore widely used in various high-speed and low-speed copy machines. However, when the surface of a heat roller contacts the surface of the visible image, the toner is likely to cause a so-called "offset phenomenon," wherein the toner is adhered to the surface of the heat roller, and thus transferred to a subsequent transfer paper. In order to prevent this phenomenon, the surface of a heat roller is coated with a material having excellent release properties for the toner, such as a fluororesin, and further a releasing agent such as a silicone oil is applied thereon. However, the method of applying a silicone oil necessitates a larger-scale fixing device, which is not only expensive but also complicated, which in turn may undesirably cause various problems.
Although processes for improving the offset phenomenon by unsymmetrizing or crosslinking the resins have been disclosed in Japanese Patent Examined Publication No. 57-493 and Japanese Patent Laid-Open Nos. 50-44836 and 57-37353, the fixing temperature has not yet been improved by these processes.
Since the lowest fixing temperature of a toner is generally between the temperature of low-temperature offsetting of the toner and the temperature of the high-temperature offsetting thereof, the serviceable temperature range of the toner is from the lowest fixing temperature to the temperature for high-temperature offsetting. Accordingly, by lowering the lowest fixing temperature as much as possible and raising the temperature at which high-temperature offsetting occurs as much as possible, the serviceable fixing temperature can be lowered and the serviceable temperature range can be widened, which enables energy saving, high-speed fixing and prevention of curling of paper.
From the above reasons, the development of a toner having excellent fixing ability and offset resistance has always been desired.
A method has been proposed to achieve low-temperature fixing by using an encapsulated toner comprising a core material and a shell formed thereon so as to cover the surface of the core material.
Among such toners, those having a core material made of a low-melting wax which is easily plastically deformable, as described in U.S. Pat. No. 3,269,626, Japanese Patent Examined Publication Nos. 46-15876 and 44-9880, and Japanese Patent Laid-Open Nos. 48-75032 and 48-75033, are poor in fixing strength and therefore can be used only in limited areas, although they can be fixed only by pressure.
Further, with respect to toners having a liquid core material, when the strength of the shell is low, the toners tend to break in the developing device and stain the inside thereof, though they can be fixed only by pressure. On the other hand, when the strength of the shell is high, a higher pressure is necessitated in order to break the capsule, thereby giving images that are too glossy. Thus, it has been difficult to control the strength of the shell.
Further, there has been proposed, as a toner for heat-and-pressure fixing, an encapsulated toner for heat roller fixing which comprises a core material made of a resin having a low glass transition temperature which serves to enhance the fixing strength, though blocking at a high temperature may take place if used alone, and a shell of a high-melting point resin wall which is formed by interfacial polymerization for the purpose of imparting a blocking resistance to the toner. However, in Japanese Patent Laid-Open No. 61-56352, this toner cannot fully exhibit the performance of the core material, because the melting point of the shell material is too high and also the shell is too tough and not easily breakable. On the same line of thinking as that described above, encapsulated toners for heat roller fixing with an improved fixing strength of the core material have been proposed (see Japanese Patent Laid-Open Nos. 58-205162, 58-205163, 63-128357, 63-128358, 63-128359, 63-128360, 63-128361, and 63-128362). However, since these toners are prepared by a spray drying method, a higher load to the equipments for the production thereof becomes necessary. In addition, they cannot fully exhibit the performance of the core material, because they have not come up with a solution for the problems in the shell.
Therefore, an encapsulated toner using a compound having thermal dissociation property as a shell material (Japanese Patent Laid-Open No. 4-212169) and an encapsulated toner using an amorphous polyester as a shell material have been proposed (Japanese Patent Laid-Open No. 6-130713).
When a thermoplastic resin such as an amorphous polyester is used as a shell material, in order to sufficiently exhibit a good fixing performance of the core material, the following methods may be employed. For instance, a thickness of the shell is reduced. Alternatively, a resin having a narrow molecular weight distribution and a low softening point may be used in the production of a toner so as to quickly melt the resin at a fixing temperature of the toner. However, when the thickness of the shell is reduced, a toner having a core material surface partly exposed without being fully covered with the shell-forming material is likely to be produced, thereby making the amount control of the shell-forming material difficult. Also, in the case of using the resin quickly melting in a narrow temperature range, where the copying speed and printing speed are low, the resin melts excessively, so that it is likely to be adhered onto a fixing roller. Therefore, offset phenomenon undesirably takes place, due to the shell material properties. As a result, the thermoplastic resin to be used as a shell material has to have a sufficient elasticity in a given fixing temperature range.